A two-time Pro Bowler, Lett was a member of the Cowboys teams that won three Super Bowls during the 1990s. He is also remembered for his involvement in two botched plays; a fumble just before scoring a touchdown in Super Bowl XXVII and a failed ball recovery after a blocked field goal in a Thanksgiving game. Neither play proved to be detrimental to the Cowboys, however, who won the Super Bowl in both seasons the plays occurred in. After retiring, Lett began a career in coaching, and rejoined the Cowboys as a coach in 2011.

During his two years at Emporia, the defense held opponents to 4.1 yards-per-play and 3.1 yards-per-carry. He earned All-NAIA District 10 recognition as a senior and finished second in the team in sacks (4), but pro scouts lost interest when he missed his first three games with a right knee injury and wasn't invited to any senior All-Star games or the NFL Scouting Combine.[2]

Lett was selected by the Dallas Cowboys in the seventh round of the 1991 NFL Draft. As a rookie, he was placed on the injured reserve list with a lower back problem on August 27. He was re-activated on November 21 and played as a reserve defensive tackle during the last five games of the season and in the playoffs. After entering the league at 6'6" and 260 pounds, by the end of his rookie year, he had put on 16 more pounds.

In his second season, he blossomed into a key player in the Cowboys' defensive line rotation and his teammates nicknamed him "Big Cat" in reference to his agility. Playing mainly as a defensive right tackle in passing situations, he finished the regular season with 19 quarterback pressures (tied for second on the team) and four passes deflected (led the team). In the postseason, he was one of the defense's best players, tying for the team lead with two tackles for losses and leading the team with three forced fumbles. In Super Bowl XXVII, he nearly set the record for the longest fumble return in Super Bowl history (64 yards). However, Lett prematurely began to celebrate and the ball was knocked out of his hand by Bills receiver Don Beebe which resulted in a touch back. He also sacked Frank Reich on the game’s final play.

In 1993, he fractured his right ankle in the third game of the season, re-injuring a fracture suffered during offseason workouts. Upon his return after missing 5 games, he played the four positions along the defensive line at different times. After playing in 11 games, he led all defensive linemen in passes deflected (4) for the second straight season. In the divisional playoff game against the Green Bay Packers, he registered 6 tackles (1 for loss), 2 passes deflected (one was intercepted), 3 quarterback pressures and recovered a fumble. In Super Bowl XXVIII, besides having 6 tackles, he forced Thurman Thomas to fumble in the third quarter, leading to a James Washington 46 yards return for a touchdown, that tied the game at 13-13 and changed the momentum of the game.

In 1994, he started all 16 games and was named to his first Pro Bowl after finishing with 68 tackles, 4 sacks and 26 quarterback pressures (second on the team).

In 1995, he served a four-game suspension in the middle of the season, before returning strong during the team drive to Super Bowl XXX, registering in the last four regular season games 22 tackles, 2 sacks, 12 quarterback pressures, a forced fumble and a recovered fumble.

Lett would be suspended at different times for violating the NFL substance abuse policy, missing a total of 28 games during his career. He served and additional full 16-game suspension (3 games in 1996 and 13 games of 1997) and an eight-game suspension at the start of the 1999 season.

In 1998, he earned his second Pro Bowl selection after starting 15 games, recording 51 tackles, 20 quarterback pressures (second on the team), 7 tackles for loss (led the team) and 4 sacks (third on the team).

His last season in Dallas was 2000, when he played in 9 games (7 starts), registered 35 tackles (14 solo), 2.5 sacks, 4 passes deflected and one forced fumble. He sprained his left medial collateral ligament during a November practice and was inactive for five games, before being placed on the injured reserve list on December 14.

When Lett was playing, he was one of the most feared defensive players in the game.[3] Even though he was constantly double teamed, he dominated at the point of attack affecting both the running and the passing game of teams. He was part of the Super Bowl winning teams in 1992, 1993 and 1995.

In 2001 he signed as a free agent with the Denver Broncos, where he played his final season as a reserve player, before being waived on March 1, 2002. Lett retired with 22.5 career quarterback sacks, 361 tackles, 128 quarterback pressures, 37 tackles for loss, 26 passes deflected, four blocked kicks and seven fumble recoveries in 121 games.

Lett was a talented player and a cornerstone of the Cowboys defense during his tenure, but he is also remembered for being involved in two infamous plays. Two of the top three of ESPN's "25 Biggest Sports Blunders" are attributed to Lett. The fans ranked him #1 and #3, whereas an expert panel placed him at #2 and #3.

The first play (ranked #1 in the ESPN fan list, #2 in the ESPN expert panel) occurred in January 1993, in Super Bowl XXVII. Late in the fourth quarter, Lett recovered a fumble on the Dallas 35-yard line and ran it back towards the end zone. When he reached the 10-yard line, Lett slowed and held the ball out as he approached the goal line. However, Lett did not see Bills player Don Beebe chasing him down from behind. Beebe knocked the ball out of Lett's outstretched hand just before he crossed the goal line, which sent the ball through the endzone, and resulted in a touchback that cost Lett his touchdown. Lett later said he was watching the Jumbotron, and trying to do a "Michael Irvin", where he put the ball out across the goal line.[4]

The Cowboys had a commanding 52–17 lead at the time, and the play is viewed as not having significantly affected the outcome of the game, but it embarrassed Lett and is still well known today. Lett's gaffe also cost the Cowboys the record for most points scored in a Super Bowl (55, by the San Francisco 49ers in Super Bowl XXIV is the current record).

The second play (ranked #3 in both ESPN lists) occurred during the next season and was more serious as it resulted in a Cowboy defeat. On Thanksgiving Day in 1993, during a rare snow and sleet storm in Dallas, the Cowboys, who came into the game with a 7–3 record, were leading the 8–2 Miami Dolphins by a score of 14–13 with 15 seconds remaining in the game. The Dolphins attempted a 41-yard field goal to take the lead but the kick was blocked by Jimmie Jones. While most of his teammates began celebrating, Lett attempted to recover the ball. He slipped on the ice as he tried to pick up the football, and Miami recovered the "muff" on the Dallas one-yard line. Had Lett simply done nothing, the Cowboys would have automatically received possession and could have run out the clock. By touching the ball and then failing to hold onto it, Lett enabled the Dolphins to take possession and then try another field goal with three seconds left on the clock. This second attempt was successful and the Dolphins won the game 16–14 as the clock expired.

The play did not hurt the Cowboys' season as they won all of their remaining regular-season games and went on to win the Super Bowl. The Dolphins took over sole possession of the lead in the AFC East with the win, but did not win another game for the rest of the year, dropping their last five games and finishing out of the playoffs. In 2008, the game was named the third-most memorable in the history of Texas Stadium by ESPN.[5]

Lett started his coaching career in 2009 as a volunteer assistant at the University of Nevada, Las Vegas. On May 9, 2009, Lett graduated from UNLV with a degree in university studies and a concentration in sociology and history. On December 19, 2009, he was appointed defensive tackle coach for the University of Louisiana at Monroe.[6]

On March 31, 2011, he was invited as part of the NFL Minority Coaching Fellowship Program to assist Dallas Cowboys defensive line coach Brian Baker during training camp. On July 20, 2011, he was hired as a full-time assistant defensive line coach for the Cowboys.[7]